Capture images from webcam

I want to be able to capture the current image of the webcam this ability is to be encapsulated in a dll. the code should be written in c++. No use of 3rd party libraries, that leaves WIA or DirectShow, which is best I dont know. When I say best I mean fastest. And it needs to run on Vista and XP, both x86 and x64 versions. The code should be thoroughly commented, so I can understand whats happening.